The present invention relates to a purge gas concentration estimation apparatus.
In a vehicle using gasoline as fuel, a canister (a carbon or charcoal canister) is normally used as an evaporative fuel treatment apparatus, in order to prevent evaporated fuel in a fuel tank from discharging into the atmosphere. The canister performs the function of adsorbing and desorbing the evaporated fuel as follows: the evaporated fuel generated from the fuel tank in an engine halt state is adsorbed by an adsorbent which is made of the activated carbon. After that, by negative pressure generated by an intake of the engine, through the canister at engine start-up, an inside of the canister is purged with atmospheric air introduced from an air port. That is, the adsorbed evaporated fuel is desorbed from the adsorbent, and is burnt in the engine. The adsorbent therefore regains its adsorbing capability by the purge, and thus being able to adsorb the evaporated fuel repeatedly and properly.
Recently, emission control, i.e. control of allowable harmful or toxic gas in exhaust gas, has become increasingly restrictive. On the other hand, in areas typified by North America, laws and regulations, which require retention of large-volume evaporated fuel gas, are enforced. For this reason, an amount of the evaporated fuel gas, which is desorbed (purged) from the canister and should be burnt in the engine, is increased, and therefore an engine air-fuel ratio control has become extremely difficult and sophisticated.
Thus, normally, when starting the purge of the canister, a small quantity of purge (pre-purge) is first performed after judging whether an engine operating condition meets the purge. And an evaporated fuel amount in the purge gas is estimated from a value of variation of an emission or exhaust air-fuel ratio between presence and absence of the purge which is detected by an oxygen sensor etc. provided in an exhaust passage. Then according to the estimated evaporated fuel amount in the purge gas, a fuel injection quantity at an execution of the purge is corrected or compensated.
However, such manner requires time before the purge is executed. And besides, it is extraordinarily difficult to meet (to control) a fuel injection condition (a fuel injection quantity) from a fuel injection valve in accordance with a change of HC (hydrocarbon) concentration (concentration of the evaporated fuel) in the purge gas after starting the purge, which is profoundly affected by an evaporated fuel retention state, an evaporated fuel retention amount and so on in the canister.
For this problem, in Japanese Patent Document 1 (Patent No. 3216276), degree of saturation of the canister (a ratio of an adsorption amount of the evaporated fuel at a measurement to an adsorption amount of the evaporated fuel at saturation of canister) is calculated from an air-fuel ratio signal after the evaporated fuel purged from the canister is burnt in the engine, a canister inside temperature obtained by a temperature sensor that is built in the canister and so on. Then, from this degree of saturation, a concentration change of a purge gas concentration (a fuel concentration in the purge gas) during the purge is estimated.
Further, Japanese Patent Document 2 (Japanese Patent Provisional Publication No. 7-253037) discloses a technique of correcting a fuel injection quantity in accordance with a fuel amount (evaporated fuel) in the purge gas, which is calculated by a detection value detected by a concentration sensor that is built in the canister and a purge gas volume flow rate calculated on the basis of this detection value by the concentration sensor and an engine temperature.